diff --git a/DBSyn/DatabaseModel.swift b/DBSyn/DatabaseModel.swift
index d3ef1b2..e108e8c 100644
--- a/DBSyn/DatabaseModel.swift
+++ b/DBSyn/DatabaseModel.swift
@@ -162,6 +162,77 @@ final class DatabaseModel: ObservableObject {
         }
     }
 
+    // Phase 66: compare bounded table samples without modifying either database.
+    struct DataComparisonReport {
+        let sourceRows: Int
+        let targetRows: Int
+        let truncated: Bool
+        let text: String
+    }
+
+    func compareTableData(source: String, target: String, table: String) async throws -> DataComparisonReport {
+        guard let profile = selected else {
+            throw NSError(domain: "DBSyn.DataCompare", code: 1, userInfo: [NSLocalizedDescriptionKey: "Connect to MySQL first."])
+        }
+        guard let sourceMetadata = introspection(for: source),
+              let targetMetadata = introspection(for: target),
+              sourceMetadata.tables.contains(table), targetMetadata.tables.contains(table) else {
+            throw NSError(domain: "DBSyn.DataCompare", code: 2, userInfo: [NSLocalizedDescriptionKey: "Introspect both databases and select a shared base table."])
+        }
+        let sourceColumns = (sourceMetadata.columnsByTable[table] ?? []).map(\.name)
+        let targetColumns = (targetMetadata.columnsByTable[table] ?? []).map(\.name)
+        guard !sourceColumns.isEmpty, sourceColumns == targetColumns else {
+            throw NSError(domain: "DBSyn.DataCompare", code: 3, userInfo: [NSLocalizedDescriptionKey: "The table columns must match in name and order before comparing data."])
+        }
+        func quote(_ identifier: String) -> String {
+            "`" + identifier.replacingOccurrences(of: "`", with: "``") + "`"
+        }
+        let password = PasswordStore.read(profile.id)
+        let sql = "SELECT * FROM \(quote(table)) LIMIT 501"
+        let left = try await NativeMySQLRunner.run(profile: profile, database: source, password: password, sql: sql)
+        let right = try await NativeMySQLRunner.run(profile: profile, database: target, password: password, sql: sql)
+        guard left.columns == right.columns else {
+            throw NSError(domain: "DBSyn.DataCompare", code: 4, userInfo: [NSLocalizedDescriptionKey: "Returned column layouts differ; comparison aborted."])
+        }
+        let limit = 500
+        let truncated = left.rows.count > limit || right.rows.count > limit
+        let a = Array(left.rows.prefix(limit))
+        let b = Array(right.rows.prefix(limit))
+        // JSON encoding gives unambiguous row keys, including embedded delimiters.
+        func counts(_ rows: [[String]]) -> [String: Int] {
+            var output: [String: Int] = [:]
+            for row in rows {
+                guard let bytes = try? JSONEncoder().encode(row),
+                      let key = String(data: bytes, encoding: .utf8) else { continue }
+                output[key, default: 0] += 1
+            }
+            return output
+        }
+        let leftCounts = counts(a)
+        let rightCounts = counts(b)
+        let keys = Set(leftCounts.keys).union(rightCounts.keys).sorted()
+        var lines = ["DBSyn Phase 66 — Read-only Table Data Comparison",
+                     "Source: \(source).\(table) — \(a.count) sampled rows",
+                     "Target: \(target).\(table) — \(b.count) sampled rows",
+                     "Columns: \(left.columns.joined(separator: ", "))", ""]
+        if truncated {
+            lines.append("WARNING: Truncated to 500 rows per side; without ORDER BY, MySQL does not guarantee which rows are returned. These results are not a complete or deterministic comparison.")
+            lines.append("")
+        }
+        var differences = 0
+        for key in keys {
+            let leftCount = leftCounts[key, default: 0]
+            let rightCount = rightCounts[key, default: 0]
+            if leftCount != rightCount {
+                differences += 1
+                lines.append("Source count \(leftCount), target count \(rightCount): \(key)")
+            }
+        }
+        if differences == 0 { lines.append("No differences detected in retrieved rows.") }
+        else { lines.insert("Distinct differing row values: \(differences)", at: 4) }
+        return DataComparisonReport(sourceRows: a.count, targetRows: b.count, truncated: truncated, text: lines.joined(separator: "\n"))
+    }
+
     @Published var selectedID: UUID?
     @Published var queryTabs: [WorkbenchQueryTab] = [
         WorkbenchQueryTab(
diff --git a/DBSyn/MainView.swift b/DBSyn/MainView.swift
index c3b5400..da8f53c 100644
--- a/DBSyn/MainView.swift
+++ b/DBSyn/MainView.swift
@@ -35,6 +35,7 @@ struct MainView: View {
     @State private var showingQueryPlan = false
     @State private var schemaChangesDatabase: String? = nil
     @State private var schemaCompareDatabase: String? = nil
+    @State private var dataCompareDatabase: String? = nil
     @State private var showingQueryHistory = false
     @State private var showingSavedQueries = false
     @State private var showingSaveQuery = false
@@ -98,6 +99,14 @@ struct MainView: View {
                 DBSynSchemaChangesView(model: model, database: database)
             }
         }
+        .sheet(isPresented: Binding(
+            get: { dataCompareDatabase != nil },
+            set: { if !$0 { dataCompareDatabase = nil } }
+        )) {
+            if let database = dataCompareDatabase {
+                DBSynDataCompareView(model: model, sourceDatabase: database)
+            }
+        }
         .sheet(isPresented: Binding(
             get: { schemaCompareDatabase != nil },
             set: { if !$0 { schemaCompareDatabase = nil } }
@@ -500,6 +509,7 @@ struct MainView: View {
                 }
                 Button("Review Schema Changes…") { schemaChangesDatabase = db }
                 Button("Compare Schema Structures…") { schemaCompareDatabase = db }
+                Button("Compare Table Data…") { dataCompareDatabase = db }
                 Divider()
                 Button("Set as Default Schema") { model.selectedDatabase = db }
                 Button("Filter to This Schema") { schemaFilter = db }
@@ -6542,3 +6552,114 @@ private struct DBSynSchemaCompareView: View {
         }
     }
 }
+
+
+// MARK: - Phase 66: bounded, read-only table data comparison
+private struct DBSynDataCompareView: View {
+    @ObservedObject var model: DatabaseModel
+    let sourceDatabase: String
+    @Environment(\.dismiss) private var dismiss
+    @State private var targetDatabase = ""
+    @State private var table = ""
+    @State private var isComparing = false
+    @State private var report: DatabaseModel.DataComparisonReport?
+    @State private var errorMessage: String?
+
+    private var commonTables: [String] {
+        guard let source = model.introspection(for: sourceDatabase),
+              let target = model.introspection(for: targetDatabase) else { return [] }
+        return Array(Set(source.tables).intersection(target.tables)).sorted()
+    }
+
+    var body: some View {
+        VStack(alignment: .leading, spacing: 14) {
+            HStack {
+                Text("Compare Table Data").font(.title2).bold()
+                Spacer()
+                Button("Done") { dismiss() }
+            }
+            Text("Read-only comparison of up to 500 rows per table. No data is modified.")
+                .foregroundStyle(.secondary)
+            HStack {
+                Text("Source: \(sourceDatabase)")
+                Spacer()
+                Picker("Target database", selection: $targetDatabase) {
+                    Text("Choose database").tag("")
+                    ForEach(model.databases.filter { $0 != sourceDatabase }, id: \.self) { name in
+                        Text(name).tag(name)
+                    }
+                }.frame(width: 340)
+            }
+            HStack {
+                Picker("Table", selection: $table) {
+                    Text("Choose table").tag("")
+                    ForEach(commonTables, id: \.self) { name in Text(name).tag(name) }
+                }.frame(width: 420)
+                Button("Refresh Metadata") {
+                    model.introspect(database: sourceDatabase)
+                    if !targetDatabase.isEmpty { model.introspect(database: targetDatabase) }
+                }
+                Spacer()
+                Button("Compare") { compare() }
+                    .disabled(table.isEmpty || targetDatabase.isEmpty || isComparing)
+            }
+            if isComparing { ProgressView("Comparing table samples…") }
+            if let errorMessage {
+                Text(errorMessage).foregroundStyle(.red).textSelection(.enabled)
+            }
+            if let report {
+                HStack {
+                    Text("Source: \(report.sourceRows) rows sampled")
+                    Text("Target: \(report.targetRows) rows sampled")
+                    Spacer()
+                    Button("Copy Report") {
+                        NSPasteboard.general.clearContents()
+                        NSPasteboard.general.setString(report.text, forType: .string)
+                    }
+                }.font(.caption)
+                if report.truncated {
+                    Label("At least one table exceeded 500 rows. Differences apply only to the retrieved samples; this is NOT a complete comparison.", systemImage: "exclamationmark.triangle")
+                        .foregroundStyle(.orange)
+                }
+                ScrollView {
+                    Text(report.text)
+                        .font(.system(.body, design: .monospaced))
+                        .textSelection(.enabled)
+                        .frame(maxWidth: .infinity, alignment: .leading)
+                        .padding(12)
+                }
+                .background(Color(nsColor: .textBackgroundColor))
+            } else if !isComparing && errorMessage == nil {
+                ContentUnavailableView("Select two databases and a table", systemImage: "tablecells", description: Text("Introspect both databases to list their shared base tables."))
+            }
+            Spacer(minLength: 0)
+        }
+        .padding(20)
+        .frame(minWidth: 820, minHeight: 520)
+        .onAppear {
+            if targetDatabase.isEmpty {
+                targetDatabase = model.databases.first(where: { $0 != sourceDatabase }) ?? ""
+            }
+        }
+        .onChange(of: targetDatabase) { _, _ in table = ""; report = nil }
+        .onChange(of: table) { _, _ in report = nil }
+    }
+
+    private func compare() {
+        isComparing = true
+        errorMessage = nil
+        report = nil
+        let source = sourceDatabase
+        let target = targetDatabase
+        let selectedTable = table
+        Task {
+            do {
+                let value = try await model.compareTableData(source: source, target: target, table: selectedTable)
+                report = value
+            } catch {
+                errorMessage = error.localizedDescription
+            }
+            isComparing = false
+        }
+    }
+}
diff --git a/docs/PHASE66-DATA-COMPARISON.md b/docs/PHASE66-DATA-COMPARISON.md
new file mode 100644
index 0000000..b7d6943
--- /dev/null
+++ b/docs/PHASE66-DATA-COMPARISON.md
@@ -0,0 +1,21 @@
+# Phase 66 — Advanced Data Comparison (bounded preview)
+
+The Schemas navigator now provides **Compare Table Data…** on each database.
+Select another database on the same connection and a shared base table, then click Compare.
+
+The comparison is **read-only**. It compares exact returned row values, including duplicates,
+using counts of canonical JSON-encoded row arrays. Both databases must have matching column
+names and order in their introspection snapshots. Views are excluded.
+
+## Important limitations
+
+- A maximum of **500 rows per database** are compared. An extra row is retrieved to detect truncation.
+- Without a deterministic ordering, MySQL may return different subsets on repeated queries.
+- No primary-key alignment or update detection is implemented in this phase; changed records
+  appear as removed/added row values.
+- The native MySQL protocol currently represents result cells as strings, so SQL NULL and
+  a literal string may not be distinguishable. Do not use this comparison as a data-integrity audit.
+- For large datasets, Phase 67 should implement paginated, key-aware comparison with safe
+  conflict handling before any data synchronisation is offered.
+
+This phase does not modify database records or create synchronization SQL.